1. Technical Field
The present invention relates to a novel cyanate ester compound, a curable resin composition containing the novel cyanate ester compound, and a cured product thereof. More particularly, the present invention relates to a novel cyanate ester compound used for a resin composition being in a liquid form at ordinary temperature and capable of improving a thermal expansion rate of a cured product.
2. Background Art
In recent years, there have been many electronic devices having light, thin, short and compact as keywords, such as a mobile phone, an ultraslim liquid crystal TV and plasma TV, and a lightweight laptop in the field of a semiconductor-related material. Therefore, very high performance has been required for a package material. Particularly, a tip package has a complicated structure, which in turn increases the number of elements that cannot easily be sealed by sealing other than liquid sealing. For example, it is necessary to partially seal an element having a cavity-down-type structure such as EBGA but transfer molding cannot handle the element. For such a reason, development of a highly-functional liquid curable resin material as a sealing material has been required.
In case of the liquid sealant, because of the difficulty of highly filling of a filler and raising of Tg (glass transition temperature) of a matrix resin itself unlike a powder sealing material, the coefficient of thermal expansion of the sealing material tends to be increased. Therefore, the liquid sealing material has solder heat resistance and heat shock resistance inferior to those of the powder sealing material subjected to transfer molding. As a result, a crack in a resin or a chip is more likely to be generated by a stress generated by the difference between the coefficient of thermal expansion of the liquid sealing material and the coefficient of thermal expansion of a chip, which disadvantageously reduces the reliability of a semiconductor device. Therefore, a resin for a liquid sealing material, which has high Tg and a low coefficient of thermal expansion, has been required.
Epoxy resin compositions containing a bisphenol A-based epoxy resin or an alicyclic epoxy resin or the like as a main component, a liquid acid anhydride or phenol novolac as a curing agent, and an additive such as an inorganic filler are proposed as liquid sealing resin compositions for sealing a semiconductor element (for example, see Patent Documents 1, 2, and 3). However, the resin composition containing the bisphenol A-based epoxy resin or the alicyclic epoxy resin or the like as the main component has low Tg and a larger coefficient of thermal expansion in a high temperature range. These resin compositions also have a large dielectric constant and dielectric loss in a high-frequency region, which do not necessarily satisfy the requirements of miniaturization, high density, and speeding up of the semiconductor device.
On the other hand, a cyanate ester resin has been known through the ages as a thermosetting resin having excellent heat resistance as well as a low dielectric constant and low dielectric loss. Particularly, a resin composition using a bisphenol A-based cyanate ester resin and a bismaleimide compound in combination, as proposed in Patent Document 4, is referred to as a BT resin. Since the BT resin has excellent electrical characteristics, mechanical characteristics, and chemical resistance or the like, the BT resin is suitable for a sealing material of the semiconductor element. However, because the bisphenol A-based cyanate ester is a crystalline compound having a melting point of 80° C., the bisphenol A-based cyanate ester cannot be used as it is as a liquid sealing material, and it is necessary to use the bisphenol A-based cyanate ester and other component being in a liquid form at ordinary temperature in combination. However, the combined use of other component is influenced by the added component and reduces the degree of freedom of blending of the composition, which may hinder functional improvement.
For example, Patent Document 5 discloses a resin composition containing a cyanate ester compound using a triphenylmethane-based cyanate ester compound to improve thermal expansibility. However, the triphenylmethane-based cyanate ester compound is a solid at ordinary temperature, and is insufficient for the liquid sealing material. Furthermore, Patent Document 6 discloses that a difunctional cyanatophenyl-based cyanate ester compound in which two cyanatophenyl groups are bonded via an asymmetric alkylene group has a low viscosity and non-crystallinity, and a resin cured product using the compound has an excellent heat deflection temperature and flexural strength. Patent Document 6 discloses bis(4-cyanatophenyl)-2,2-propane, bis(4-cyanatophenyl)-1,1-ethane, and bis(4-cyanatophenyl)-2,2-butane or the like as examples thereof.